When does cytosine dimerize? a protonation window for the charge- and resonance-assisted [CytH+…Cyt] synthon

Abstract

This study defines an operational protonation window for selecting the cytosinium–cytosine [CytH⁺…Cyt] synthon in nitrocarboxylate salts. By combining X-ray diffraction, ITC and DFT, we show that dimer formation is not a simple consequence of cytosine protonation, but requires coexistence of neutral and mono-protonated cytosine, modulated by proton activity, stoichiometry and hydration.